Nickel-containing steel for low temperature

ABSTRACT

A nickel-containing steel for low temperature according to an aspect of the present invention has a chemical composition within a predetermined range, in which a metallographic structure of a thickness middle portion contains 2.0 vol % to 20.0 vol % of an austenite phase, an average grain size of prior austenite grains is 3.0 μm to 15.0 μm, an average aspect ratio of the prior austenite grains is 1.0 to 2.4, a plate thickness is 4.5 mm to 30 mm, the chemical composition and the average grain size of the prior austenite grains are further limited depending on the plate thickness, a yield stress at room temperature is 460 MPa to 710 MPa, and a tensile strength at the room temperature is 560 MPa to 810 MPa.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No. 16/758,010 filed on Apr. 21, 2020, which is the National Phase of PCT International Application No. PCT/JP2017/038615, filed on Oct. 26, 2017, all of which are hereby expressly incorporated by reference into the present application.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a nickel-containing steel for low temperature, that is, a steel containing nickel (Ni) suitable for low temperature at around −253° C.

RELATED ART

In recent years, expectations for the use of liquid hydrogen as clean energy have increased. A steel plate used for a tank that stores and transports a liquefied gas such as liquid hydrogen requires excellent low temperature toughness, and austenitic stainless steel which is less likely to undergo brittle fracture has been used. Although austenitic stainless steel has sufficient low temperature toughness, the yield stress of a general-purpose material at room temperature is about 200 MPa.

In a case where austenitic stainless steel with insufficient strength is applied to a liquid hydrogen tank, there is a limit to the increase in the size of the tank. Furthermore, when the yield stress of the steel is about 200 MPa, the plate thickness thereof needs to exceed 30 mm when the tank is increased in size. Therefore, an increase in the weight of the tank and an increase in manufacturing cost are problems. For such problems, for example, an austenitic high Mn stainless steel having a plate thickness of 5 mm and a 0.2% proof stress of 450 MPa or more at room temperature is proposed (for example, refer to Patent Document 1).

Ferritic 9% Ni steel has been used for a tank for a liquefied natural gas (LNG) (sometimes referred to as an LNG tank) which is representative of liquefied gas. Although LNG has a higher temperature than liquid hydrogen, 9% Ni steel has excellent low temperature toughness, and in the related art, various 9% Ni steels and 7% Ni steels suitable for LNG tanks have been proposed (for example, refer to Patent Documents 2 to 4). Moreover, 9% Ni steel can also have a yield strength of 590 MPa or more at room temperature, and can also be applied to a large LNG tank.

For example, Patent Document 2 discloses a steel for low temperature with a plate thickness of 25 mm, which contains 5% to 7.5% of Ni, has a yield stress of more than 590 MPa at room temperature, and a brittle fracture surface ratio of 50% or less in a Charpy test at −233° C. In Patent Document 2, low temperature toughness is secured by setting the volume fraction of residual austenite stable at −196° C. to 2% to 12%.

In addition, Patent Document 3 discloses a steel for low temperature with a plate thickness of 6 mm to 50 mm, which contains 5% to 10% of Ni, has a yield stress of 590 MPa or more at room temperature, and has excellent low temperature toughness at −196° C. after strain aging. In Patent Document 3, low temperature toughness after strain aging is secured by setting the volume fraction of residual austenite to 3% or more and the effective grain size to 5.5 μm or less, and introducing appropriate defects into the intragranular structure.

Furthermore, Patent Document 4 discloses a thin nickel steel plate for low temperature with a thickness of 6 mm, which contains 7.5% to 12% Ni and is excellent in the low temperature toughness of not only the base metal but also a welded heat-affected zone. In Patent Document 4, the Si and Mn contents are reduced so as not to generate martensite-islands constituents in the welded heat-affected zone, whereby low temperature toughness at −196° C. is secured.

PRIOR ART DOCUMENT Patent Document

-   [Patent Document 1] Japanese Patent No. 5709881 -   [Patent Document 2] Japanese Unexamined Patent Application, First     Publication No. 2014-210948 -   [Patent Document 3] Japanese Unexamined Patent Application, First     Publication No. 2011-219849 -   [Patent Document 4] Japanese Unexamined Patent Application, First     Publication No. H3-223442

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, the austenitic high Mn stainless steel disclosed in Patent Document 1 has a larger coefficient of thermal expansion than ferritic 9% Ni steel. For large liquid hydrogen tanks, 9% Ni steel with a low coefficient of thermal expansion is advantageous due to problems such as fatigue. On the other hand, as a result of examinations by the present inventors, the 9% Ni steel and 7% Ni steel disclosed in Patent Documents 2 to 4 cannot obtain sufficient toughness at −253° C., which is the liquefaction temperature of liquid hydrogen.

The present invention has been made in view of such circumstances, and an object thereof is to provide a nickel-containing steel for low temperature, which has sufficient toughness at a low temperature of around −253° C., a yield stress of 460 MPa or more at room temperature, and a tensile strength of 560 MPa or more at room temperature.

Means for Solving the Problem

The present inventors conducted numerous examinations on the toughness at a low temperature of around −253° C. and the tensile strength and yield stress at room temperature of a steel having a higher Ni content than 9% Ni steel in the related art. As a result, it was found that in order to secure low temperature toughness, it is necessary to limit the Si content, strictly limit the Mn content, and optimally controlling the volume fraction of austenite and the average grain size and average aspect ratio of prior austenite grains.

The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) According to an aspect of the present invention, a nickel-containing steel for low temperature includes, as a chemical composition, by mass %: C: 0.030% to 0.070%; Si: 0.03% to 0.30%; Mn: 0.20% to 0.80%; Ni: 10.5% to 12.4%; Al: 0.010% to 0.060%; N: 0.0015% to 0.0060%; O: 0.0007% to 0.0030%; Cu: 0% to 0.50%; Cr: 0% to 0.50%; Mo: 0% to 0.40%; Nb: 0% to 0.020%; V: 0% to 0.080%; Ti: 0% to 0.020%; B: 0% to 0.0020%; Ca: 0% to 0.0040%; REM: 0% to 0.0050%; P: 0.0080% or less; S: 0.0040% or less; and a remainder: Fe and impurities, in which a metallographic structure of a thickness middle portion contains 2.0 vol % to 20.0 vol % of an austenite phase; an average grain size of prior austenite grains measured in a section of the thickness middle portion parallel to a rolling direction and a thickness direction is 3.0 μm to 15.0 μm; an average aspect ratio of the prior austenite grains measured in the section of the thickness middle portion parallel to the rolling direction and the thickness direction is 1.0 to 2.4; a plate thickness is 4.5 mm to 30 mm; when the plate thickness is more than 20 mm, the nickel-containing steel contains Ni: 11.5% or more; when the plate thickness is 20 mm or less and the nickel-containing steel contains Ni: less than 11.5%, the nickel-containing steel contains C: 0.060% or less, Si: 0.19% or less, Mn: 0.30% to 0.50%, Al: 0.050% or less, N: 0.0050% or less, Cr: 0.35% or less, Nb: 0.015% or less, V: 0.060% or less, Ti: 0.015% or less, P: 0.0060% or less, and S: 0.0030% or less, and the average grain size of the prior austenite grains is 8.0 μm or less; a yield stress at room temperature is 460 MPa to 710 MPa, and a tensile strength at the room temperature is 560 MPa to 810 MPa.

(2) The nickel-containing steel for low temperature according to (1) may include Ni: 11.5% or more and Mn: 0.50% or less as the chemical composition.

(3) The nickel-containing steel for low temperature according to (1) or (2) may include Ni: 11.5% or more as the chemical composition, in which the average grain size of the prior austenite grains may be 9.0 μm or less.

(4) In the nickel-containing steel for low temperature according to any one of (1) to (3), an average effective grain size measured in the section of the thickness middle portion parallel to the rolling direction and the thickness direction may be 2.0 μm to 8.0 μm.

(5) In the nickel-containing steel for low temperature according to any one of (1) to (3), an average effective grain size measured in the section of the thickness middle portion parallel to the rolling direction and the thickness direction may be 2.0 μm to 5.0 μm.

Effects of the Invention

According to the present invention, it is possible to provide a nickel-containing steel for low temperature having sufficient extremely low temperature toughness for uses such as a liquid hydrogen tank, and sufficient yield stress and tensile strength at room temperature. Therefore, for example, when the nickel-containing steel for low temperature of the present invention is used in a liquid hydrogen tank, the plate thickness of a steel plate for the tank can be made thinner than that of austenitic stainless steel. Therefore, according to the present invention, it is possible to achieve an increase in the size and a reduction in the weight of the liquid hydrogen tank, an improvement in heat insulation performance by a reduction in surface area with respect to volume, an effective use of the tank site, an improvement in the fuel efficiency of a liquid hydrogen carrier, and the like. Furthermore, compared to the austenitic stainless steel, the nickel-containing steel for low temperature of the present invention has a small coefficient of thermal expansion, so that the design of a large tank can be simplified and the tank manufacturing cost can be reduced. As described above, the industrial contribution of the present invention is extremely remarkable.

EMBODIMENTS OF THE INVENTION

The toughness of a steel for low temperature in the related art (for example, 9% Ni steel) was evaluated at −165° C. or −196° C., but a toughness evaluation temperature for a nickel-containing steel for low temperature (hereinafter, simply abbreviated to “Ni steel”) according to the present embodiment is significantly lower than that of the steel in the related art.

The present inventors conducted numerous examinations in order to clarify the influence of the amounts of elements, a metallographic structure, and the like on the toughness of Ni steel at −253° C. According to the knowledge in the related art, it has been considered effective to increase the Ni content in order to increase low temperature toughness. However, as a result of examinations by the present inventors, it was found that the toughness at a low temperature is not sufficiently improved even if the amount of Ni in 9% Ni steel in the related art is simply changed to increase.

In addition, for the distinction from temperatures such as −165° C. and −196° C. and concise description, hereinafter, a temperature of around −253° C. is referred to as “extremely low temperature” for convenience.

Furthermore, the present inventors examined another method for increasing the toughness of Ni steel at an extremely low temperature (hereinafter, referred to as “extremely low temperature toughness”). As a result, it was determined that in order to secure low temperature toughness at an extremely low temperature, it is necessary to, in addition to increase the Ni content, (a) set the C content to 0.030% to 0.070%, (b) set the Si content to 0.03% to 0.30%, (c) set the Mn content to 0.20% to 0.80%, (d) set the P content to 0.0080% or less, (e) control the average grain size and average aspect ratio of prior austenite grains, and (f) control the volume fraction of an austenite phase. Furthermore, the knowledge that the low temperature toughness at an extremely low temperature is further improved by (g) controlling an average effective grain size was also obtained. Moreover, it was found that in a case where the plate thickness of Ni steel is 20 mm or less and the above-described conditions are more strictly limited, the Ni content can be slightly reduced to reduce raw material costs.

Next, a Ni steel according to the present embodiment will be described.

In addition, it is necessary to change the Ni content of the Ni steel according to the present embodiment depending on the plate thickness. In a case where the plate thickness is large (that is, the plate thickness is more than 20 mm), the cooling rate during reheating hardening becomes slow, and it becomes difficult to secure the low temperature toughness through heat treatments. Therefore, in a case where the plate thickness is more than 20 mm, the amount of Ni, which is an element for securing the low temperature toughness, has to be 11.5% or more. On the other hand, in a case where the plate thickness is 20 mm or less, it is easy to secure low temperature toughness through heat treatments, so that it is possible to impart sufficient low temperature toughness to the Ni steel while suppressing the Ni content to less than 11.5%. As a matter of course, the Ni steel according to the present embodiment may have a plate thickness of 20 mm or less and a Ni content of 11.5% or more.

However, in a case where the plate thickness is 20 mm or less and Ni is less than 11.5% (hereinafter, sometimes abbreviated to “in a case where the Ni content is low”), it is necessary to more strictly control elements that affect the low temperature toughness other than the Ni content (the amounts of C, Si, Mn, Al, N, Cr, Nb, V, Ti, P, and S, and the average grain size of prior austenite grains) compared to the case where Ni content is 11.5% or more.

Regarding the requirements that require further limitation depending on the Ni content and the plate thickness due to the above circumstances, the intent thereof will be described as appropriate.

First, the composition of the Ni steel according to the present embodiment will be described. Unless otherwise specified, % in contents means mass %.

(C: 0.030% to 0.070%)

C is an element that increases the yield stress at room temperature, and also contributes to the formation of martensite and austenite. When the C content is less than 0.030%, strength cannot be secured, and the extremely low temperature toughness of the Ni steel may decrease due to the formation of coarse bainite. Therefore, the lower limit of the C content of the Ni steel is set to 0.030%. A preferable lower limit of the C content is 0.035%.

On the other hand, when the C content exceeds 0.070%, cementite is likely to precipitate at prior austenite grain boundaries, and this cementite causes fracture at the grain boundaries, thereby reducing the extremely low temperature toughness of the Ni steel. Therefore, the upper limit of the C content is set to 0.070%. The upper limit of the C content is preferably 0.060%, more preferably 0.050%, and even more preferably 0.045%.

In a case where the Ni content is small, the C content needs to be 0.060% or less. In a case where the Ni content is small, a preferable upper limit of the C content is 0.055%, 0.050%, or 0.045%. The lower limit and preferable lower limit of the C content of a Ni steel with a small Ni content may be the same as those of a Ni steel having a Ni content of 11.5% or more.

(Si: 0.03% to 0.30%) Si is an element that increases the yield stress at room temperature. When the Si content is less than 0.03%, the effect of improving the yield stress at room temperature is small. Therefore, the lower limit of the Si content of the Ni steel is set to 0.03%. A preferable lower limit of the Si content is 0.05%.

On the other hand, when the Si content exceeds 0.30%, cementite at the prior austenite grain boundaries is likely to be coarsened, and this cementite causes fracture at the grain boundaries, thereby reducing the extremely low temperature toughness of the Ni steel. Therefore, limiting the upper limit of the Si content to 0.30% is extremely important in order to secure the toughness at an extremely low temperature. The upper limit of the Si content is preferably 0.20% and more preferably 0.15%, and the upper limit of the Si content is even more preferably 0.10%.

In a case where the Ni content is small, the Si content needs to be 0.19% or less. In a case where the Ni content is small, a preferable upper limit of the Si content is 0.16%, 0.13%, or 0.10% or less. The lower limit and preferable lower limit of the Si content of a Ni steel with a small Ni content may be the same as those of a Ni steel having a Ni content of 11.5% or more.

(Mn: 0.20% to 0.80%)

Mn is an element that increases the yield stress at room temperature. When the Mn content is less than 0.20%, strength cannot be secured, and extremely low temperature toughness may decrease due to the formation of coarse bainite and the like. Therefore, the lower limit of the Mn content is set to 0.20%. A preferable lower limit of the Mn content is 0.30%, 0.28%, or 0.25%.

On the other hand, when the Mn content exceeds 0.80%, Mn segregated at the prior austenite grain boundaries and MnS precipitated coarsely cause fractures at the grain boundaries, resulting in a decrease in the extremely low temperature toughness. Therefore, limiting the upper limit of the Mn content to 0.80% is extremely important in order to secure the toughness at an extremely low temperature. The upper limit of the Mn content is preferably 0.60%, and more preferably 0.50%.

In a case where the Ni content is small, the Mn content needs to be 0.30% to 0.50%. In a case where the Ni content is small, a preferable lower limit of the Mn content is 0.35% or more. In a case where the Ni content is small, a preferable upper limit of the Mn content is 0.45% or 0.40%.

(Ni: 10.5% to 12.4%)

Ni is an essential element for securing the extremely low temperature toughness. When the Ni content is less than 10.5%, the toughness at an extremely low temperature is insufficient. Therefore, the lower limit of the Ni content is set to 10.5%. A preferable lower limit of the Ni content is 10.8%, 11.0%, or 11.5%. On the other hand, Ni is an expensive element, and when Ni is contained in more than 12.4%, the economy is impaired. Therefore, the Ni content is limited to 12.4% or less. The upper limit of the Ni content may be set to 12.2%, 12.0%, or 11.8%. In a case where the plate thickness is 20 mm or less, the upper limit of the Ni content may be set to 11.3%, 11.1%, or 10.9%.

In a case where the plate thickness is more than 20 mm, the Ni content needs to be 11.5% or more. In a case where the plate thickness is more than 20 mm, a preferable lower limit of the Ni content is 11.8% or 12.0%. The upper limit and preferable upper limit of the Ni content of a Ni steel having a plate thickness of more than 20 mm may be the same values as those of a Ni steel having a plate thickness of 20 mm or less.

(Al: 0.010% to 0.060%)

Al is an element mainly used for deoxidation. In addition, Al forms AlN and contributes to the refinement of the metallographic structure and a reduction in the amount of solute N, which lowers the extremely low temperature toughness. When the Al content is less than 0.010%, the effect of deoxidation, the effect of the refinement of the metallographic structure, and the effect of reducing the amount of solute N are small. Therefore, the lower limit of the Al content is set to 0.010% or less. The lower limit of the Al content is preferably 0.015% or more, and more preferably 0.020% or more.

However, when the Al content exceeds 0.060%, the toughness at an extremely low temperature decreases. Therefore, the upper limit of the Al content is set to 0.060%. A more preferable upper limit of the Al content is 0.040% or 0.035%.

In a case where the Ni content is small, the Al content needs to be 0.050% or less. In a case where the Ni content is small, a preferable upper limit of the Al content is 0.040% or 0.020%. The lower limit and preferable lower limit of the Al content of a Ni steel with a small Ni content may be the same as those of a Ni steel having a Ni content of 11.5% or more.

(N: 0.0015% to 0.0060%)

N contributes to the formation of nitrides that refine grains. When the N content is reduced to less than 0.0015%, fine AlN that suppresses the coarsening of the austenite grain size during a heat treatment is insufficient, and there are cases where the austenite grains become coarse and the extremely low temperature toughness of the Ni steel decreases. For this reason, the N content may be set to 0.0015% or more, and preferably 0.0020% or more, or 0.0025% or more.

On the other hand, when the N content exceeds 0.0060%, the amount of solute N increases or coarsening of AlN occurs, resulting in the decrease in the toughness at an extremely low temperature. For this reason, the N content is set to 0.0060% or less, preferably 0.0050% or less, and more preferably 0.0040% or less or 0.0035% or less.

In a case where the Ni content is small, the N content needs to be 0.0050% or less. In a case where the Ni content is small, a preferable upper limit of the N content is 0.0040% or 0.0030%. The lower limit and preferable lower limit of the N content of a Ni steel with a small Ni content may be the same as those of a Ni steel having a Ni content of 11.5% or more.

(O: 0.0007% to 0.0030%)

O is an impurity, and when the O content exceeds 0.0030%, there are cases where Al₂O₃ clusters increase and the toughness at an extremely low temperature decreases. Therefore, the upper limit of the O content is set to 0.0030%. The upper limit of the O content is preferably 0.0025%, more preferably 0.0020%, and even more preferably 0.0015%. Although it is desirable that the O content is small, there are cases where a reduction in the O content to less than 0.0007% is accompanied by an increase in cost. Therefore, the lower limit of the 0 content is set to 0.0007%. The lower limit of the O content may be set to 0.0008% or 0.0010%. The upper and lower limits and preferable upper and lower limits of the O content are the above values regardless of the plate thickness and the Ni content.

(P: 0.0080% or Less)

P is an element that causes grain boundary embrittlement at the prior austenite grain boundaries and is thus harmful to the extremely low temperature toughness. Therefore, it is desirable that the P content is small. When the P content exceeds 0.0080%, there are cases where the toughness at an extremely low temperature decreases. Therefore, the P content is limited to 0.0080% or less. The upper limit of the P content is preferably 0.0060%, more preferably 0.0040%, and even more preferably 0.0030%.

There are cases where P is incorporated into molten steel as an impurity during the manufacturing of the molten steel. However, the lower limit thereof does not need to be particularly limited, and the lower limit thereof is 0%. However, when the P content is excessively reduced, there are cases where the manufacturing cost increases. Therefore, the lower limit of the P content may be 0.0002%, 0.0005%, or 0.0008%.

In a case where the Ni content is small, the P content needs to be 0.0060% or less. In a case where the Ni content is small, a preferable upper limit of the P content is 0.0050%, 0.0040%, or 0.0030%. The lower limit and preferable lower limit of the P content of a Ni steel with a small Ni content may be the same as those of a Ni steel having a Ni content of 11.5% or more.

(S: 0.0040% or Less)

S forms MnS, which becomes a brittle fracture origin in some cases, and is thus an element harmful to the extremely low temperature toughness. When the S content exceeds 0.0040%, there are cases where the toughness at an extremely low temperature decreases. Therefore, the S content is limited to 0.0040% or less. The upper limit of the S content is preferably 0.0030%, more preferably 0.0020%, and even more preferably 0.0010%.

There are cases where S is incorporated into molten steel as an impurity during the manufacturing of the molten steel. However, the lower limit thereof does not need to be particularly limited, and the lower limit thereof is 0%. However, when the S content is excessively reduced, there are cases where the manufacturing cost increases. Therefore, the lower limit of the S content may be 0.0002%, 0.0005%, or 0.0008%.

In a case where the Ni content is small, the S content needs to be 0.0030% or less. In a case where the Ni content is small, a preferable upper limit of the S content is 0.0010%, 0.0015%, or 0.0010%. The lower limit and preferable lower limit of the S content of a Ni steel with a small Ni content may be the same as those of a Ni steel having a Ni content of 11.5% or more.

(Cu: 0% to 0.50%)

Cu is an element that increases the yield stress at room temperature, so that the Ni steel according to the present embodiment may contain Cu. However, when the Cu content exceeds 0.50%, the toughness at an extremely low temperature decreases. Therefore, the upper limit of the Cu content is set to 0.50%. The upper limit of the Cu content is preferably 0.40%, more preferably 0.30%, and even more preferably 0.20%.

There are cases where Cu is incorporated as an impurity into the Ni steel during the manufacturing of molten steel. However, the lower limit thereof does not need to be particularly limited, and the lower limit thereof is 0%. The lower limit of the Cu content may be set to 0.02%, 0.05%, or 0.10%. The upper and lower limits and preferable upper and lower limits of the Cu content are the above values regardless of the plate thickness and the Ni content.

(Cr: 0% to 0.50%)

Cr is an element that increases the yield stress at room temperature, so that the Ni steel according to the present embodiment may contain Cr. However, when the Cr content exceeds 0.50%, the toughness at an extremely low temperature decreases. Therefore, the upper limit of the Cr content is set to 0.35%. The upper limit of the Cr content is preferably 0.30%, more preferably 0.20%, and even more preferably 0.10%.

There are cases where Cr is incorporated as an impurity into the Ni steel during the manufacturing of molten steel. However, the lower limit thereof does not need to be particularly limited, and the lower limit thereof is 0%. The lower limit of the Cr content may be set to 0.02%, 0.05%, or 0.10%.

In a case where the Ni content is small, the Cr content needs to be 0.35% or less. In a case where the Ni content is small, a preferable upper limit of the Cr content is 0.30%, 0.25%, or 0.20%. The lower limit and preferable lower limit of the Cr content of a Ni steel with a small Ni content may be the same as those of a Ni steel having a Ni content of 11.5% or more.

(Mo: 0% to 0.40%)

Mo is an element that increases the yield stress at room temperature and also has an effect of suppressing grain boundary embrittlement, so that the Ni steel according to the present embodiment may contain Mo. However, Mo is an expensive element, and when the Mo content exceeds 0.40%, the economy is impaired. Therefore, the Mo content is limited to 0.40% or less.

There are cases where Mo is incorporated as an impurity into the Ni steel during the manufacturing of molten steel. However, the lower limit thereof does not need to be particularly limited, and the lower limit thereof is 0%. The lower limit of the Mo content may be set to 0.02%, 0.05%, or 0.10%. The upper and lower limits and preferable upper and lower limits of the Mo content are the above values regardless of the plate thickness and the Ni content.

(Nb: 0% to 0.020%)

Nb is an element that increases the yield stress at room temperature, and also has an effect of improving the extremely low temperature toughness by refining the metallographic structure, so that the Ni steel according to the present embodiment may contain Nb. However, when the Nb content exceeds 0.020%, the toughness at an extremely low temperature decreases. Therefore, the upper limit of the Nb content is set to 0.020%. The upper limit of the Nb content is preferably 0.015%, and more preferably 0.010%.

There are cases where Nb is incorporated as an impurity into the Ni steel during the manufacturing of molten steel. However, the lower limit thereof does not need to be particularly limited, and the lower limit thereof is 0%. The lower limit of the Nb content may be set to 0.002%, or 0.005%

In a case where the Ni content is small, the Nb content needs to be 0.015% or less. In a case where the Ni content is small, a preferable upper limit of the Nb content is 0.012% or 0.010%. The lower limit and preferable lower limit of the Nb content of a Ni steel with a small Ni content may be the same as those of a Ni steel having a Ni content of 11.5% or more.

(V: 0% to 0.080%)

V is an element that increases the yield stress at room temperature, so that the Ni steel according to the present embodiment may contain V. However, when the V content exceeds 0.080%, the toughness at an extremely low temperature decreases. Therefore, the upper limit of the V content is set to 0.080%. The upper limit of the V content is preferably 0.060%, and more preferably 0.040%.

There are cases where V is incorporated as an impurity into the Ni steel during the manufacturing of molten steel. However, the lower limit thereof does not need to be particularly limited, and the lower limit thereof is 0%. The lower limit of the V content may be set to 0.002%, 0.005%, or 0.010%.

In a case where the Ni content is small, the V content needs to be 0.060% or less. In a case where the Ni content is small, a preferable upper limit of the V content is 0.050% or 0.040%. The lower limit and preferable lower limit of the V content of a Ni steel with a small Ni content may be the same as those of a Ni steel having a Ni content of 11.5% or more.

(Ti: 0% to 0.020%)

Ti forms TiN and contributes to the refinement of the metallographic structure and a reduction in the amount of solute N that lowers the extremely low temperature toughness, so that the Ni steel according to the present embodiment may contain Ti. However, when the Ti content exceeds 0.020%, the toughness at an extremely low temperature decreases. Therefore, the upper limit of the Ti content is set to 0.020%. The upper limit of the Ti content is preferably 0.015%, and more preferably 0.010%.

There are cases where Ti is incorporated as an impurity into the Ni steel during the manufacturing of molten steel. However, the lower limit thereof does not need to be particularly limited, and the lower limit thereof is 0%. The lower limit of the Ti content may be set to 0.001%, 0.002%, or 0.005%.

In a case where the Ni content is small, the Ti content needs to be 0.015% or less. In a case where the Ni content is small, a preferable upper limit of the Ti content is 0.012% or 0.010%. The lower limit and preferable lower limit of the Ti content of a Ni steel with a small Ni content may be the same as those of a Ni steel having a Ni content of 11.5% or more.

The upper and lower limits and preferable upper and lower limits of the amounts of B, Ca, REM, Sb, Sn, As, Co, Zn, and W described below are the same regardless of the plate thickness and Ni content.

(B: 0% to 0.0020%)

B is an element that increases the yield stress at room temperature, and also contributes to a reduction in the amount of solute N, which lowers the extremely low temperature toughness, by forming BN. Therefore, the Ni steel according to the present embodiment may contain B. However, when B is contained in more than 0.0020%, the toughness at an extremely low temperatures decreases, so that the upper limit of the B content is set to 0.0020%. The upper limit of the B content is preferably 0.0015%, more preferably 0.0012%, and even more preferably 0.0010%.

There are cases where B is incorporated as an impurity into the Ni steel during the manufacturing of molten steel. However, the lower limit thereof does not need to be particularly limited, and the lower limit thereof is 0%. The lower limit of the B content may be set to 0.0001%, 0.0002%, or 0.0005%.

(Ca: 0% to 0.0040%)

Ca causes MnS, which is an inclusion that tends to be stretched by hot rolling and thus easily increases the harmfulness to the extremely low temperature toughness, to be spheroidized as CaS, thereby being effective in improving the extremely low temperature toughness. Therefore, the Ni steel according to the present embodiment may contain Ca. However, when the Ca content exceeds 0.0040%, oxysulfides containing Ca are coarsened, and these oxysulfides lower the toughness of the Ni steel at an extremely low temperature. Therefore, the upper limit of the Ca content is limited to 0.0040%, and is preferably set to 0.0030%.

There are cases where Ca is incorporated as an impurity into the Ni steel during the manufacturing of molten steel. However, the lower limit thereof does not need to be particularly limited, and the lower limit thereof is 0%. The lower limit of the Ca content may be set to 0.0005%, 0.0010%, or 0.0015%.

(REM: 0% to 0.0050%)

REM (rare-earth metal element) refers to a total of 17 elements composed of Sc, Y, and lanthanoids, and the amount of REM means the total amount of these 17 elements. Like Ca, REM causes MnS, which is an inclusion that tends to be stretched by hot rolling and thus easily increases the harmfulness to the extremely low temperature toughness, to be spheroidized as an oxysulfide of REM, thereby being effective in improving the extremely low temperature toughness. Therefore, the Ni steel according to the present embodiment may contain REM. However, when the REM content exceeds 0.0050%, oxysulfides containing REM are coarsened, and these oxysulfides lower the toughness of the Ni steel at an extremely low temperature. Therefore, the upper limit of the REM content is limited to 0.0050%, and is preferably set to 0.0040%.

There are cases where REM is incorporated as an impurity into the Ni steel during the manufacturing of molten steel. However, the lower limit thereof does not need to be particularly limited, and the lower limit thereof is 0%. The lower limit of the REM content may be set to 0.0005%, 0.0010%, or 0.0015%.

The Ni steel according to the present embodiment contains or limits the above-mentioned elements, and the remainder consists of iron and impurities. Here, the impurities mean elements that are incorporated into the Ni steel due to various factors in the manufacturing process, including raw materials such as ore and scrap, when steel is industrially manufactured, and are allowed in a range in which the Ni steel according to the present embodiment is not adversely affected. However, in the Ni steel according to the present embodiment, it is necessary to define the upper limits of P and S among the impurities as described above. In addition to the above-mentioned elements, the Ni steel according to the present embodiment may contain, for example, the following alloying elements for the purpose of further improving the strength of the steel material itself, the extremely low temperature toughness, and the like, or as impurities from auxiliary raw materials such as scrap.

Sb impairs the extremely low temperature toughness. Therefore, the Sb content is preferably 0.005% or less, more preferably 0.003% or less, and most preferably 0.001% or less.

Sn impairs the extremely low temperature toughness. Therefore, the Sn content is preferably 0.005% or less, more preferably 0.003% or less, and most preferably 0.001% or less.

As impairs the extremely low temperature toughness. Therefore, the As content is preferably 0.005% or less, more preferably 0.003% or less, and most preferably 0.001% or less.

Moreover, in order to fully exhibit the effects of the above-described elements, it is preferable to limit the amount of each of Co, Zn, and W to 0.01% or less or 0.005% or less.

There is no need to limit the lower limits of Sb, Sn, As, Co, Zn, and W, and the lower limit of each of the elements is 0%. Moreover, even if an alloying element (for example, P, S, Cu, Cr, Mo, Nb, V, Ti, B, Ca, and REM) with no defined lower limit or with a lower limit of 0% is intentionally added or incorporated as an impurity, when the amount thereof is within the above-described range, the steel (steel material) for low temperature is interpreted as the Ni steel according to the present embodiment.

Next, the metallographic structure of the Ni steel according to the present embodiment will be described. In addition, regarding the requirements that require further limitation depending on the Ni content and the plate thickness, the intent thereof will be described as appropriate.

The present inventors newly found that fracture occurs at the prior austenite grain boundaries at an extremely low temperature, and toughness is likely to decrease. The Ni steel according to the present embodiment is manufactured by performing water cooling or air cooling after hot rolling and performing heat treatments including reheating hardening, an intermediate heat treatment, and tempering. The prior austenite grain boundaries mentioned here are grain boundaries of austenite that have existed during heating of the reheating hardening. A large proportion of prior austenite grain boundaries that have existed during the heating of the reheating hardening are coarse. It is considered that Mn, P, and Si are segregated at the coarse prior austenite grain boundaries, and these elements lower the bonding force of the prior austenite grain boundaries and impair the toughness at −253° C.

Although prior austenite grain boundaries are newly generated during the intermediate heat treatment, in the manufacturing of the Ni steel according to the present embodiment, the temperature of the intermediate heat treatment is as low as 610° C. to 650° C., and there are very few coarse prior austenite grains newly generated. Since the amount of Mn, P, and Si segregated at prior austenite grain boundaries which are not coarse is relatively small, fracture from the prior austenite grain boundaries (most of which are prior austenite grain boundaries generated during the intermediate heat treatment) which are not coarse is relatively unlikely to occur.

Therefore, in order to secure the extremely low temperature toughness, it is substantially important to control the coarse prior austenite grains. Therefore, in the Ni steel according to the present embodiment, in a case of measuring the grain size or aspect ratio of the prior austenite grains, only coarse prior austenite grains are measurement objects. In the present embodiment, whether or not the prior austenite grain boundaries are coarse is determined based on whether or not the grain size of the prior austenite grains is 2.0 μm or more. That is, the prior austenite grains having a grain size of less than 2.0 μm are determined to be prior austenite grains that do not impair the extremely low temperature toughness. When the average grain size or average aspect ratio of the prior austenite grains is measured, the prior austenite grains having a grain size of less than 2.0 μm are excluded. In the Ni steel according to the present embodiment, the “average grain size of the prior austenite grains” means the average value of the grain sizes of prior austenite grains having a grain size of 2.0 μm or more, and the “average aspect ratio of the prior austenite grains” means the average value of the aspect ratios of prior austenite grains having a grain size of 2.0 μm or more.

The present inventors conducted numerous examinations on methods for suppressing fracture at the prior austenite grain boundaries at an extremely low temperature. As a result, the present inventors found that fracture at the prior austenite grain boundaries is suppressed and the toughness at an extremely low temperature can be secured when six conditions are simultaneously satisfied including (A) setting the C content to 0.070% or less (corresponding to a case where the Ni content is 11.5% or more, and the same applies to (B) to (F)), (B) setting the Mn content to 0.80% or less, (C) setting the P content to 0.0080% or less, (D) setting the Si content to 0.30% or less, (E) causing the average grain size of the prior austenite grains to be 15.0 μm or less, and (F) suppressing the average aspect ratio of the prior austenite grains to 2.4 or less. In addition, when the plate thickness is caused to be 20 mm or less and the conditions (A) to (F) are more strictly controlled, the Ni content can be reduced to less than 11.5%, so that raw material costs can be reduced.

As described above, it is presumed that at an extremely low temperature, fracture is likely to occur selectively in a portion where the bonding force is relatively weak, such as a coarse prior austenite grain boundary. Therefore, it is considered that the decrease in the bonding force of the coarse prior austenite grain boundaries can be suppressed by suppressing precipitation of cementite and segregation of Mn and P that weakens the bonding force of the coarse prior austenite grain boundaries. Moreover, an increase in the C content and the Si content and coarsening of the prior austenite grains promote the coarsening of intergranular cementite. Therefore, the suppression of the C content and the Si content and the refinement of the prior austenite are necessary for suppressing the fracture at the prior austenite grain boundaries at an extremely low temperature.

(Average Grain Size of Prior Austenite Grains Measured in Section of Thickness Middle Portion Parallel to Rolling Direction and Thickness direction: 3.0 μm to 15.0 μm)

The average grain size of the prior austenite grains measured in a section of a thickness middle portion parallel to a rolling direction and a thickness direction needs to be 3.0 μm to 15.0 μm. In the present embodiment, unless otherwise specified, the average grain size of the prior austenite grains indicates an average grain size measured in the section of the thickness middle portion parallel to the rolling direction and the thickness direction. When the average grain size of the prior austenite grains is more than 15.0 μm, cementite precipitated at the prior austenite grain boundaries becomes coarse, and the concentration of Mn and P at the grain boundaries increases. Precipitation of coarse cementite and concentration of Mn and P weaken the bonding force of the prior austenite grain boundaries and cause fractures at the prior austenite grain boundaries in some cases. In addition, there are cases where segregation points of Mn and P and coarse cementite become brittle fracture origins. As described above, since an increase in the average grain size of the prior austenite grains decreases the toughness at an extremely low temperature, the upper limit of the average grain size of the prior austenite grains is set to 15.0 μm. The upper limit of the average grain size of the prior austenite grains may be set to 12.0 μm, 10.0 μm, 9.0 μm, 8.0 μm, or 7.5 μm. In order to refine the average grain size of the prior austenite grains to less than 3.0 μm, methods accompanied by an increase in manufacturing cost such as an increase in the number of heat treatments are necessary. Therefore, the lower limit of the average grain size of the prior austenite grains is set to 3 μm.

In a case where the Ni content is small, the average grain size of the prior austenite grains needs to be 8.0 μm or less. The upper limit thereof may be set to 7.0 μm or 6.0 μm as necessary. The lower limit of the average grain size of the prior austenite grains of a Ni steel having a small Ni content may be the same as that of a Ni steel having a Ni content of 11.5% or more.

(Average Aspect Ratio of Prior Austenite Grains Measured in Section of Thickness Middle Portion Parallel to Rolling Direction and Thickness Direction: 1.0 to 2.4)

Furthermore, in the Ni steel according to the present embodiment, the average aspect ratio of the prior austenite grains is 2.4 or less. In the present embodiment, unless otherwise specified, the average aspect ratio of the prior austenite grains indicates an average aspect ratio measured in the section of the thickness middle portion parallel to the rolling direction and the thickness direction. The average aspect ratio of the prior austenite grains is the ratio between the length and thickness of the prior austenite grains in the section (L-section) parallel to the rolling direction and the thickness direction, that is, the length of the prior austenite grains in the rolling direction/the thickness of the prior austenite grains in the thickness direction. Therefore, the lower limit of the average aspect ratio is in a case where the length and thickness of the prior austenite grains are the same, and the average aspect ratio is 1.0 or more.

The above numerical value range of the average aspect ratio of the prior austenite grains is achieved in a case where a manufacturing method, which will be described below, is applied to the steel having the above-described chemical composition. The upper limit of the average aspect ratio of the prior austenite grains may be set to 2.3, 2.2, 2.0, 1.8, or 1.7. The upper and lower limits and preferable upper and lower limits of the average aspect ratio of the prior austenite grains are the above values regardless of the plate thickness and the Ni content.

The average grain size and the average aspect ratio of the prior austenite grains are measured using the section (L-section) of the thickness middle portion parallel to the rolling direction and the thickness direction as an observed section. The prior austenite grain boundaries are revealed by corroding the observed section with a saturated aqueous solution of picric acid. An enlarged photograph of the L-section subjected to the corrosion treatment is photographed at five or more visual fields with a scanning electron microscope (SEM) at a magnification of 1,000-fold or 2,000-fold. The circle equivalent grain sizes (diameters) of at least 20 prior austenite grains having a circle equivalent grain size (diameter) of 2.0 μm or more, which are included in these SEM photographs, are obtained by image processing, and the average value thereof is calculated, thereby obtaining the average grain size of the prior austenite grains measured in the section of the thickness middle portion parallel to the rolling direction and the thickness direction. In addition, the ratios (aspect ratios) between the length in the rolling direction and the thickness in the thickness direction of at least 20 prior austenite grains having a circle equivalent grain size (diameter) of 2.0 μm or more, which are included in the above SEM photographs, are measured, and the average value thereof is calculated, thereby obtaining the average aspect ratio of the prior austenite grains measured in the section of the thickness middle portion parallel to the rolling direction and the thickness direction. In a case where prior austenite grains having a grain size of less than 2.0 μm are included, the above-described measurement is performed excluding the prior austenite grains.

(Volume Fraction of Austenite Phase in Metallographic Structure of Thickness Middle Portion: 2.0 vol % to 20.0 vol %)

In order to increase the toughness at an extremely low temperature, the metallographic structure of the thickness middle portion of the Ni steel at room temperature contains an austenite phase in a volume fraction of 2.0 vol % or more. In the present embodiment, unless otherwise specified, the volume fraction of the austenite phase indicates a volume fraction measured at the thickness middle portion. This austenite phase is different from prior austenite and is an austenite phase present in a Ni steel after a heat treatment, and the volume fraction thereof is measured by an X-ray diffraction method. In a case where 2.0 vol % to 20.0 vol % of the austenite phase is contained in the thickness middle portion of the Ni steel at room temperature, it is considered that a stable austenite phase is present in the Ni steel in an amount that is necessary for securing the toughness at an extremely low temperature even when cooled to an extremely low temperature.

It is considered that in a case where an austenite phase which is stable even at an extremely low temperature is present, applied stress and strain are relieved by the plastic deformation of austenite, and thus toughness is improved. In addition, the austenite phase is relatively uniformly and finely generated at the prior austenite grain boundaries, the block boundaries and lath boundaries of tempered martensite, and the like. That is, it is considered that the austenite phase is present in the vicinity of a hard phase, which is likely to be a brittle fracture origin, relieves the concentration of stress or strain around the hard phase, and thus contributes to the suppression of the occurrence of brittle fracture. Furthermore, it is considered that as a result of generating 2.0 vol % or more of the austenite phase in the thickness middle portion, coarse cementite, which becomes a brittle fracture origin, can be significantly reduced. The lower limit of the volume fraction of the austenite phase in the metallographic structure of the thickness middle portion may be set to 3.0 vol % or 4.0 vol %.

On the other hand, when the volume fraction of the austenite phase increases excessively, the concentration of C or the like into the austenite phase becomes insufficient, and the possibility of transformation of the austenite phase into martensite at an extremely low temperature increases. There are cases where unstable austenite that transforms into martensite at an extremely low temperature reduces the extremely low temperature toughness at an extremely low temperature. Therefore, the volume fraction of the austenite phase in the metallographic structure of the thickness middle portion is preferably 20.0 vol % or less, or 15.0 vol % or less. The upper limit of the volume fraction of the austenite phase in the metallographic structure of the thickness middle portion may be set to 12.0 vol %, 10.0 vol %, or 6.0 vol %.

In a case where the Ni content is small, the volume fraction of the austenite phase is preferably set to 6.0 vol % or less. As necessary, the upper limit thereof may be set to 5.0 vol %, 4.5 vol %, or 4.0 vol %.

The remainder of the metallographic structure of the Ni steel according to the present embodiment is mainly (tempered) martensite. In order to manufacture a Ni steel in which the prior austenite grain size and the average aspect ratio of prior austenite grains are within the above-described ranges, it is necessary for the manufacturing method to include water cooling or air cooling after hot rolling, reheating hardening, an intermediate heat treatment, and tempering. In a case where such a manufacturing method is applied to a steel having the above-described chemical composition, the remainder of the obtained metallographic structure (that is, the primary phase) is necessarily tempered martensite. In addition, there are cases where the remainder of the metallographic structure contains a phase (for example, coarse inclusions) which is not classified as either austenite or tempered martensite. In a case where the total volume fraction of the austenite phase and the tempered martensite phase in the metallographic structure of the thickness middle portion is 99.0% or more, the inclusion of phases other than these is allowed. In a case of measuring the volume fraction of the tempered martensite phase, the area fraction measured by microstructure observation with an optical microscope using nital as a corrosive solution is used as the volume fraction as it is (this is because the area fraction is basically the same as the volume fraction).

The volume fraction of the austenite phase in the thickness middle portion is measured by taking a sample having a surface parallel to the plate surface of the Ni steel from the thickness middle portion of the Ni steel and applying an X-ray diffraction method to the sample. The volume fraction of the austenite phase is obtained from the ratio between the integrated intensities of austenite (face-centered cubic structure) and tempered martensite (body-centered cubic structure) of X-ray peaks. Specifically, the taken sample is subjected to X-ray diffraction, and the volume fraction of the austenite phase may be measured from the ratio between the integrated intensities of the (111) plane, (200) plane, and (211) plane of an a phase having a BCC structure and the integrated intensities of the (111) plane, (200) plane, and (220) plane of an austenite phase having a FCC structure.

In the present invention, a treatment (so-called deep cooling treatment) for cooling a test piece to an extremely low temperature is unnecessary before the measurement of the volume fraction of the austenite phase. However, in a case where only a test piece after being subjected to a deep cooling treatment is present, the volume fraction of the austenite phase may be measured using the test piece after being subjected to the deep cooling treatment.

(Average Effective Grain Size Measured in Section of Thickness Middle Portion Parallel to Rolling Direction and Thickness Direction: Preferably 2.0 μm to 8.0 μm)

An average effective grain size measured in the section of the thickness middle portion parallel to the rolling direction and the thickness direction (hereinafter abbreviated to “average effective grain size”) is preferably set to 2.0 μm to 8.0 μm. In the present embodiment, an effective grain size is defined as the circle equivalent diameter of a region (effective grain) surrounded by a boundary of a metallographic structure having an orientation difference of 15° or more. In the present embodiment, unless otherwise specified, the average effective grain size indicates an average effective grain size measured in the section of the thickness middle portion parallel to the rolling direction and the thickness direction. When the effective grain size is refined, resistance to propagation of fracture cracks increases and the toughness is improved. However, in order to refine the average effective grain size to less than 2.0 μm, methods accompanied by an increase in manufacturing cost such as an increase in the number of heat treatments are necessary. Therefore, the lower limit of the average effective grain size is preferably set to 2.0 μm. The lower limit of the average effective grain size may be set to 3.0 μm, 4.0 μm, or 5.0 μm. On the other hand, when the average effective grain size is more than 8.0 μm, there are cases where stress exerted on hard phases that become the brittle fracture origins, that is, inclusions such as coarse cementite, coarse AlN, MnS, and alumina in the prior austenite grain boundaries and tempered martensite increases, and the toughness at an extremely low temperature decreases. Therefore, the upper limit of the average effective grain size is preferably set to 8.0 μm. The upper limit of the average effective grain size may be set to 7.0 μm, 6.0 μm, or 5.0 μm.

In a case where the Ni content is small, the average effective grain size is preferably set to 5.0 μm or less. The lower limit and preferable lower limit of the average effective grain size of a Ni steel having a small Ni content may be the same as those of a Ni steel having a Ni content of 11.5% or more.

The average effective grain size is measured by using an electron backscatter diffraction (EBSD) analyzer attached to a scanning electron microscope, with the section (L-section) of the thickness middle portion parallel to the rolling direction and the thickness direction as an observed section. Observation of five or more visual fields is performed at a magnification of 2,000-fold, a boundary of a metallographic structure having an orientation difference of 15° or more is regarded as a grain boundary, grains surrounded by the grain boundaries are regarded as effective grains, the circle equivalent grain sizes (diameters) of the effective grains are obtained by image processing, and the average value of the circle equivalent grain sizes are calculated, thereby obtaining the average effective grain size measured in the section of the thickness middle portion parallel to the rolling direction and the thickness direction.

(Plate Thickness: 4.5 mm to 30 mm)

The Ni steel according to the present embodiment is mainly a Ni steel plate, and the plate thickness thereof is 30 mm or less. A Ni steel with a plate thickness of less than 4.5 mm is rarely used as a material for a large scale structure such as a liquid hydrogen tank, so that the lower limit of the plate thickness is set to 4.5 mm. In a case where the plate thickness is more than 30 mm, the cooling rate during the reheating hardening is extremely slow, and it is very difficult to secure the low temperature toughness in the compositional range of the Ni steel according to the present application (particularly, the Ni content). As necessary, the lower limit of the plate thickness may be set to 6 mm, 8 mm, 10 mm, or 12 mm, and the upper limit of the plate thickness may be set to 25 mm, 20 mm, or 16 mm.

(Yield Stress at Room Temperature: 460 MPa to 710 MPa)

(Tensile Strength at Room Temperature: 560 MPa to 810 MPa)

The yield stress of the Ni steel according to the present embodiment at room temperature is set to 460 MPa to 710 MPa. In addition, the tensile strength of the Ni steel according to the present embodiment at room temperature is set to 560 MPa to 810 MPa. The lower limit of the yield stress may be set to 470 MPa, 500 MPa, or 520 MPa. The upper limit of the yield stress may be set to 690 MPa, 670 MPa, or 650 MPa. The lower limit of the tensile strength may be set to 580 MPa, 600 MPa, or 620 MPa. The upper limit of the tensile strength may be set to 780 MPa, 760 MPa, or 750 MPa. In the present embodiment, the room temperature is set to 20° C. in principle.

Next, a method of manufacturing the steel for low temperature according to the present embodiment will be described. If the Ni steel according to the present embodiment has the above-described configuration regardless of the manufacturing method, the effect can be obtained. However, for example, according to the following manufacturing method, the Ni steel according to the present embodiment can be obtained stably.

The method of manufacturing the Ni steel according to the present embodiment includes: a step of adjusting the amounts of elements in a state in which the temperature of molten steel is set to 1650° C. or lower, the concentration of O in the molten steel is set to 0.01% or less, and the concentration of S in the molten steel is set to 0.02% or less, and thereafter manufacturing a steel piece by continuous casting; a step of heating the obtained steel piece to 950° C. to 1160° C. and retaining the steel piece for 30 minutes to 180 minutes; a step of performing hot rolling on the steel piece under the condition that a cumulative rolling reduction at 950° C. or lower is 80% or more and 95% or less and a finishing temperature is 650° C. to 850° C.; a step of performing water cooling on the hot-rolled steel plate to room temperature with a cooling start temperature of 550° C. to 850° C. or performing air cooling on the hot-rolled steel plate; a step of performing reheating hardening on the hot-rolled steel plate at a reheating hardening temperature of 720° C. to 880° C. for a retention time of 20 minutes to 180 minutes with a water cooling finishing temperature of 200° C. or lower; a step of performing an intermediate heat treatment on the hot-rolled steel plate at an intermediate heat treatment temperature of 610° C. to 650° C. for a retention time of 20 minutes to 180 minutes; and a step of performing tempering on the hot-rolled steel plate at a tempering temperature of 530° C. to 570° C. for a retention time of 20 minutes to 180 minutes. These manufacturing conditions are preferably further limited according to the Ni content and the like.

Hereinafter, details of the manufacturing conditions will be described.

Before the hot rolling, a slab is heated. Here, the heating temperature is set to 950° C. to 1160° C. When the heating temperature of the slab is lower than 950° C., there are cases where the heating temperature is lower than a predetermined hot rolling finishing temperature. When the heating temperature of the slab exceeds 1160° C., austenite grain sizes become coarse during the heating, and the toughness of the Ni steel at an extremely low temperature may decrease. Due to solutionizing of AlN, the retention time of the heating is 30 minutes to 180 minutes. In a case where the Ni content is small, the heating temperature of the hot rolling is set to 950° C. to 1100° C. Even in a case where the Ni content is small, the retention time of the heating is 30 minutes to 180 minutes.

In the hot rolling, when the cumulative rolling reduction at 950° C. or lower is less than 80%, refinement of austenite grains by recrystallization of austenite in the slab during the rolling is insufficient, and there are cases where a portion of the austenite grains after the rolling is coarsened and the extremely low temperature toughness of the Ni steel decreases. Therefore, the lower limit of the cumulative rolling reduction at 950° C. or lower is set to 80%. Homogenous refinement of prior austenite grains by recrystallization during rolling is particularly important in securing the extremely low temperature toughness of the Ni steel according to the present embodiment, and strict restriction on the rolling temperature and the cumulative rolling reduction is required. On the other hand, when the cumulative rolling reduction at 950° C. or lower exceeds 95%, the rolling time becomes long and problems occur in productivity in some cases, so that the upper limit of the cumulative rolling reduction at 950° C. or lower is set to 95%.

When the finishing temperature of the hot rolling is lower than 650° C., deformation resistance increases and the load on a rolling mill increases. Therefore, the lower limit of the finishing temperature of the hot rolling is set to 650° C. When the finishing temperature of the hot rolling exceeds 850° C., dislocations introduced by rolling are reduced due to recovery and there are cases where the extremely low temperature toughness of the Ni steel is insufficient, or there are cases where the yield stress of the Ni steel at room temperature is insufficient. Therefore, the upper limit of the finishing temperature is set to 850° C. In a case where the Ni content is small, the upper limit of the finishing temperature is set to 800° C.

Although cooling performed after the hot rolling may be either water cooling or air cooling, it is preferable to perform water cooling. In the case of water cooling, it is preferable that the water cooling finishing temperature is set to 200° C. or lower, and the water cooling start temperature is set to 550° C. to 850° C.

The reheating hardening is a heat treatment in which the hot-rolled steel plate is heated to a reheating hardening temperature, retained at the reheating hardening temperature, and then cooled, and is effective for refinement of prior austenite. The reheating hardening temperature (heating temperature during reheating) is set to 720° C. to 880° C. When the reheating hardening temperature is lower than 720° C., a portion that does not transform into austenite remains in the hot-rolled steel plate after being subjected to the reheating hardening, and there are cases where the yield stress or tensile strength of the Ni steel at room temperature decreases. When the reheating hardening temperature exceeds 880° C., the austenite grain size becomes coarse and AlN becomes coarse, so that the extremely low temperature toughness of the Ni steel decrease. The retention time during the reheating hardening is set to 20 minutes to 180 minutes. When the retention time of the reheating hardening is shorter than 20 minutes, there is concern that austenitic transformation may not proceed sufficiently. On the other hand, in a case where the retention time of the reheating hardening is longer than 180 minutes, there is concern that austenite grains may become coarse. As cooling during the reheating hardening, tempering to 200° C. or lower from the heating temperature during the reheating is performed. In a case where the Ni content is small, the reheating hardening temperature is set to 740° C. to 780° C. The average cooling rate during the hardening is set to 10° C./s or more.

The intermediate heat treatment is a heat treatment in which the hot-rolled steel plate after being subjected to the reheating hardening is heated to the intermediate heat treatment temperature, retained at the intermediate heat treatment temperature, and then cooled, and is effective for refinement of the effective grain size, which contributes to the improvement of the extremely low temperature toughness and securing of the austenite phase. The intermediate heat treatment temperature is set to 610° C. to 650° C. When the intermediate heat treatment temperature is lower than 610° C., austenitic transformation becomes insufficient, and the fraction of tempered martensite which is excessively tempered increases, so that there are cases where the strength of the Ni steel decreases. Furthermore, when the intermediate heat treatment temperature is lower than 610° C., there are cases where the extremely low temperature toughness of the Ni steel decreases.

When the intermediate heat treatment temperature exceeds 650° C., the austenitic transformation in the hot-rolled steel plate proceeds excessively. As a result, stabilization of austenite is insufficient, and an austenite phase having a volume fraction of 2.0 vol % or more may not be secured in the thickness middle portion of the Ni steel or the extremely low temperature toughness of the Ni steel may decrease. The retention time of the intermediate heat treatment is set to 20 minutes to 180 minutes. In a case where the retention time of the intermediate heat treatment is shorter than 20 minutes, there is concern that the austenitic transformation does not proceed sufficiently. On the other hand, in a case where the retention time of the intermediate heat treatment is longer than 180 minutes, there is concern that carbides adversely affecting the toughness may be precipitated. A cooling method during the intermediate heat treatment is water cooling in order to avoid tempering embrittlement, and water cooling to 200° C. or lower is performed. The average cooling rate during the water cooling is set to 8° C./s or more.

The tempering is a heat treatment in which the hot-rolled steel plate after being subjected to the intermediate heat treatment is heated to the tempering temperature, retained at the tempering temperature, and then cooled, and is effective for securing the austenite phase. The tempering temperature is set to 530° C. to 570° C. When the tempering temperature is lower than 530° C., the austenite phase in the thickness middle portion of the Ni steel cannot be secured in a volume fraction of 2.0 vol % or more, and there are cases where the extremely low temperature toughness of the Ni steel is insufficient. When the tempering temperature exceeds 570° C., there are cases where the amount of the austenite phase of the Ni steel at room temperature exceeds 20.0 vol % in terms of volume fraction. When this Ni steel is cooled to an extremely low temperature, a portion of austenite is transformed into high C martensite, and there are cases where the extremely low temperature toughness of the Ni steel decreases. For this reason, the upper limit of the tempering temperature is 570° C. or lower. The retention time of the tempering is set to 20 minutes to 180 minutes. In a case where the retention time of the tempering is shorter than 20 minutes, there is concern that the stability of austenite may not be sufficiently secured. On the other hand, in a case where the retention time of the tempering is longer than 180 minutes, there is concern that carbides adversely affecting the toughness may be precipitated, and the tensile strength may be insufficient. A cooling method during the tempering is water cooling in order to avoid tempering embrittlement, and water cooling to 200° C. or lower is performed. The average cooling rate during the water cooling is set to 5° C./s or more.

EXAMPLES

Examples of the present invention will be described below. However, the following examples are examples of the present invention, and the present invention is not limited to the examples described below.

Example 1: Ni Steel Having Ni Content of 11.5% or More

Steel was melted by a converter and slabs having a thickness of 150 mm to 300 mm were manufactured by continuous casting. Tables 1 and 2 show the chemical compositions of Kinds of steel A1 to A24. These slabs were heated, subjected to controlled rolling, directly subjected to water cooling or air cooling, and subjected to heat treatments including reheating hardening, an intermediate heat treatment, and tempering, whereby steel plates were manufactured. The retention time of the heating of the hot rolling was set to 30 minutes to 120 minutes, and the retention time of the heat treatments including the reheating hardening, the intermediate heat treatment, and the tempering was set to 20 minutes to 60 minutes. In a case of performing water cooling after the hot rolling, water cooling to 200° C. or lower was performed. Means for cooling in the heat treatments including the reheating hardening, the intermediate heat treatment, and the tempering was water cooling, and water cooling to 200° C. or lower from the treatment temperature of each of the heat treatments was performed. Samples were taken from the steel plates, and the metallographic structure, tensile properties, and toughness thereof were evaluated.

TABLE 1 Steel Chemical composition (mass %, remainder consists of Fe and impurities) material C Si Mn P S Cu Ni Cr Mo Al Nb Ti A1 0.030 0.19 0.25 0.0060 0.0010 0.05 11.8 0.03 0.03 0.025 A2 0.070 0.08 0.31 0.0040 0.0022 11.9 0.024 A3 0.033 0.30 0.36 0.0030 0.0009 12.3 0.02 0.028 0.013 A4 0.038 0.08 0.20 0.0030 0.0007 11.5 0.025 A5 0.032 0.25 0.80 0.0040 0.0008 0.04 12.2 0.017 0.006 A6 0.035 0.03 0.22 0.0020 0.0040 12.0 0.50 0.40 0.025 A7 0.063 0.11 0.45 0.0030 0.0011 12.0 0.11 0.20 0.060 A8 0.058 0.20 0.35 0.0080 0.0013 12.1 0.020 0.020 0.020 A9 0.067 0.05 0.75 0.0060 0.0013 0.50 12.4 0.010 A10 0.031 0.27 0.21 0.0030 0.0005 0.02 11.6 0.35 0.015 0.015 A11 0.052 0.07 0.60 0.0030 0.0031 11.7 0.32 0.15 0.040 0.008 A12 0.050 0.12 0.41 0.0040 0.0007 0.24 11.8 0.050 A13 0.046 0.14 0.40 0.0050 0.0007 0.43 11.7 0.033 Steel Chemical composition (mass %, remainder consists of Fe and impurities) material V B Ca REM N O Note A1 0.0025 0.0009 Present A2 0.0025 0.0015 Invention A3 0.061 0.0010 0.0026 0.0013 Example A4 0.020 0.0025 0.0012 A5 0.0005 0.0015 0.0035 0.0016 A6 0.0008 0.0040 0.0012 A7 0.0020 0.0053 0.0008 A8 0.080 0.0028 0.0018 0.0027 A9 0.0020 0.0027 0.0011 A10 0.0040 0.0040 0.0013 A11 0.045 0.0050 0.0015 0.0016 A12 0.0060 0.0007 A13 0.0020 0.0030 Blank means that no element is intentionally added.

TABLE 2 Steel Chemical composition (mass %, remainder consists of Fe and impurities) material C Si Mn P S Cu Ni Cr Mo Al Nb Ti A14 0.026 0.24 0.24 0.0030 0.0015 12.0 0.030 A15 0.077 0.26 0.26 0.0030 0.0015 12.0 0.030 A16 0.055 0.34 0.26 0.0030 0.0014 12.1 0.032 A17 0.054 0.27 0.15 0.0030 0.0010 12.0 0.031 A18 0.048 0.21 0.89 0.0030 0.0005 12.0 0.030 A19 0.049 0.22 0.21 0.0090 0.0032 0.10 12.2 0.15 0.031 A20 0.060 0.22 0.26 0.0050 0.0046 11.9 0.13 0.08 0.020 A21 0.063 0.21 0.72 0.0050 0.0009 12.0 0.61 0.020 A22 0.064 0.05 0.72 0.0050 0.0012 0.10 12.0 0.066 A23 0.036 0.05 0.71 0.0030 0.0006 12.0 0.020 0.024 A24 0.035 0.05 0.71 0.0030 0.0008 11.7 0.020 0.026 Steel Chemical composition (mass %, remainder consists of Fe and impurities) material V B Ca REM N O Note A14 0.0030 0.0014 Comparative A15 0.0030 0.0014 Example A16 0.0030 0.0014 A17 0.003 0.0029 0.0013 A18 0.0003 0.0023 0.0031 0.0015 A19 0.0045 0.0015 A20 0.0031 0.0010 A21 0.0035 0.0020 0.0015 A22 0.0025 0.0013 A23 0.0036 0.0030 0.0015 A24 0.0050 0.0074 0.0012 Blank means that no element is intentionally added. Underline means outside the range of the present invention.

The average grain size of prior austenite grains (the average grain size of prior γ) to be measured in a section of a thickness middle portion parallel to a rolling direction and a thickness direction was measured in a section (L-section) of a thickness middle portion parallel to a rolling direction and a thickness direction as an observed section. The average grain size of the prior austenite grains was measured according to JIS G 0551. First, the observed section of the sample was corroded with a saturated aqueous solution of picric acid to reveal the prior austenite grain boundaries, and thereafter five or more visual fields were photographed with a scanning electron microscope at a magnification of 1,000-fold or 2,000-fold. After identifying the prior austenite grain boundaries using the structural photographs, the circle equivalent grain sizes (diameters) of at least 20 prior austenite grains were obtained by image processing, and the average value thereof was determined as the average grain size of the prior austenite grains.

In addition, in the steel of the present invention, the refinement of the prior austenite grain size, suppression of the P content, and the like are carried out so that fracture is less likely to occur at the prior austenite grain boundaries. Therefore, it may be difficult to identify the prior austenite grain boundaries by corrosion. In such a case, after heating the sample to 450° C. to 490° C., a heat treatment of temperature retention for one hour or longer was performed, and then the average grain size of the prior austenite grains was measured by the method described above.

In a case where identification of the prior austenite grain boundaries was difficult even if the heat treatment at 450° C. to 490° C. was performed, a Charpy test piece was taken from the heat-treated sample, and the sample subjected to an impact test at −196° C. and fractured at the prior austenite grain boundaries was used. In this case, a cross section of a fracture surface at the section (L-section) parallel to the rolling direction and the thickness direction was created and corroded, and thereafter, the prior austenite grain sizes were measured by identifying the prior austenite grain boundaries of the cross section of the fracture surface of the thickness middle portion with the scanning electron microscope. When the prior austenite grain boundaries are embrittled by a heat treatment, minute cracks are generated at the prior austenite grain boundaries due to an impact load during the Charpy test, so that the prior austenite grain boundaries are easily identified.

The average aspect ratio of the prior austenite grains (average aspect ratio of prior γ grains) measured in the section of the thickness middle portion parallel to the rolling direction and the thickness direction was obtained as a ratio between the maximum value (length in the rolling direction) and the minimum value (thickness in the thickness direction) of the length of a region (prior austenite grain) surrounded by the prior austenite grain boundary identified as described above. The average aspect ratios of at least 20 prior austenite grains were measured, and the average value thereof was calculated to obtain the average aspect ratio.

The volume fraction of the austenite phase (volume fraction of γ phase) contained in the metallographic structure of the thickness middle portion was measured by taking a sample parallel to the plate surface from the thickness middle portion and performing an X-ray diffraction method thereon. The volume fraction of the austenite phase was determined from the ratio between the integrated intensities of austenite (face-centered cubic structure) and tempered martensite (body-centered cubic structure) of X-ray peaks.

The average effective grain size measured in the section of the thickness middle portion parallel to the rolling direction and the thickness direction was measured by using an EBSD analyzer attached to the scanning electron microscope, with the section (L-section) of the thickness middle portion parallel to the rolling direction and the thickness direction as an observed section. Observation of five or more visual fields was performed at a magnification of 2,000-fold, a boundary of a metallographic structure having an orientation difference of 15° or more was regarded as a grain boundary, grains surrounded by the grain boundaries were regarded as effective grains, the circle equivalent grain sizes (diameters) were obtained from the areas by image processing, and the average value of the circle equivalent grain sizes were determined as an average effective grain size.

By taking a 1A full-thickness tensile test piece specified in JIS Z 2241 whose longitudinal direction was parallel to the rolling direction (L direction), strength (yield stress and tensile strength) was measured at room temperature by the method specified in JIS Z 2241. The target value of the yield stress is 460 MPa to 580 MPa, and the target value of the tensile strength is 560 MPa to 680 MPa. The yield stress was a lower yield stress. However, there were many cases where no clear lower yield stress was observed, and in that case, the 0.2% proof stress was taken.

Regarding the extremely low temperature toughness, a CT test piece of full thickness with front and rear surfaces each ground 0.5 mm was taken in a direction (C direction) perpendicular to the rolling direction. A J-R curve was created according to the unloading compliance method specified in ASTM standard E1820-13 in liquid hydrogen (−253° C.), and a J value was converted into a Kw value. The target value of the extremely low temperature toughness is 150 MPa·√m or more.

Tables 3 and 4 show the plate thickness, manufacturing method, base metal properties, and metallographic structure of steel materials (Steel materials Nos. 1 to 32) manufactured using slabs having the chemical compositions of Kinds of steel A1 to A24.

TABLE 3 Heating, rolling, and heat treatment conditions Cumulative Inter- Heating rolling Water mediate temperature reduction Rolling cooling start Reheating heat Plate during at 950° C. finishing temperature hardening treatment Tempering Manufacturing Steel thickness rolling or lower temperature after rolling temperature temperature temperature condition No. material (mm) (° C.) (%) (° C.) (° C.) (° C.) (° C.) (° C.) a1 A1 12 1160 95 800 750 850 630 560 a2 A2 20 1100 80 830 Air cooling 830 640 550 after rolling a3 A3 20 970 92 720 680 760 630 530 a4 A4 12 980 95 790 Air cooling 720 620 530 after rolling a5 A5 30 1030 80 770 730 780 640 550 a6 A6 30 1000 91 800 Air cooling 770 620 550 after rolling a7 A7 30 1160 88 800 810 880 640 560 a8 A8 30 970 90 700 660 760 620 540 a9 A9 30 1070 85 840 810 820 640 540 a10 A9 30 1070 85 830 800 820 640 540 a11 A9 30 1070 85 830 800 820 640 540 a12 A9 30 1080 85 830 810 820 640 540 a13 A10 30 1050 88 790 760 800 640 570 a14 A11 30 1060 88 840 Air cooling 760 650 540 after rolling a15 A12 30 960 90 670 630 750 630 530 a16 A13 30 950 90 650 610 750 630 540 Metallographic structure Average Base metal properties grain Average Volume Average Extremely size of aspect fraction effective low prior γ ratio of of γ grain Yield Tensile temperature Manufacturing Steel grains prior γ phase size stress strength toughness* condition No. material (μm) grains (%) (μm) (MPa) (MPa) (MPa · √m) Note a1 A1 11.0 1.7 12.0 4.5 558 623 156 Present a2 A2 9.6 1.6 9.1 7.0 499 581 152 Invention a3 A3 4.3 2.1 7.5 2.8 527 598 165 Example a4 A4 5.2 1.8 8.5 3.2 534 624 163 a5 A5 6.8 1.9 6.3 3.0 483 570 164 a6 A6 6.3 1.7 6.8 4.7 555 663 155 a7 A7 15.0 1.0 14.0 8.6 573 673 150 a8 A8 3.5 2.2 2.4 2.6 525 613 189 a9 A9 8.2 1.4 7.6 3.8 546 645 160 a10 A9 8.3 1.4 2.0 3.7 569 665 151 a11 A9 10.5 1.2 2.1 5.6 534 641 152 a12 A9 8.2 1.4 15.0 3.8 555 645 152 a13 A10 7.5 1.8 8.0 3.5 510 602 162 a14 A11 9.3 1.5 6.7 5.1 485 580 154 a15 A12 3.4 2.3 4.5 2.3 495 580 195 a16 A13 3.3 2.4 5.6 2.0 480 567 210 *Extremely low temperature toughness is the K_(I)C value (converted from J value) in liquid hydrogen (−253° C.), the unit is MPa · √m.

TABLE 4 Heating, rolling, and heat treatment conditions Cumulative Water Inter- Heating rolling cooling mediate temperature reduction Rolling start Reheating heat Plate during at 950° C. finishing temperature hardening treatment Tempering Manufacturing Steel thickness rolling or lower temperature after rolling temperature temperature temperature condition No. material (mm) (° C.) (%) (° C.) (° C.) (° C.) (° C.) (° C.) a17 A14 30 1100 80 800 770 800 640 560 a18 A15 30 1080 85 780 750 760 620 550 a19 A16 30 1060 90 770 740 770 650 540 a20 A17 30 1050 90 760 720 750 630 550 a21 A18 30 1050 85 800 760 830 650 540 a22 A19 30 1080 85 780 750 760 630 550 a23 A20 30 1070 80 800 750 800 630 550 a24 A21 30 1070 80 790 760 780 640 560 a25 A22 30 1070 80 770 740 780 640 530 a26 A23 30 1070 80 780 740 820 630 530 a27 A24 30 1080 80 780 750 830 630 540 a28 A8 30 930 85 640 610 860 630 550 a29 A7 30 1180 88 830 800 860 640 550 a30 A7 30 1160 50 840 810 850 640 550 a31 A7 30 1130 88 880 850 840 650 560 a32 A9 30 1080 85 840 800 910 640 530 Metallographic structure Average Base metal properties grain Average Volume Average Extremely size of aspect fraction effective low prior γ ratio of of γ grain Yield Tensile temperature Manufacturing Steel grains prior γ phase size stress strength toughness* condition No. material (μm) grains (%) (μm) (MPa) (MPa) (MPa · √m) Note a17 A14 7.3 1.2 1.2 4.0 453 552 66 Comparative a18 A15 6.6 1.4 5.5 3.5 506 602 67 Example a19 A16 6.8 1.4 11.4  3.6 486 578 58 a20 A17 6.1 1.5 0.8 3.3 455 563 75 a21 A18 7.6 1.1 10.5  4.0 545 647 64 a22 A19 6.5 1.2 5.2 3.4 496 590 80 a23 A20 7.2 1.1 4.3 3.9 526 618 65 a24 A21 7.1 1.3 14.0  3.5 560 657 62 a25 A22 7.0 1.4 12.5  3.4 543 639 59 a26 A23 7.4 2.5 4.5 4.1 503 594 67 a27 A24 7.5 1.0 3.6 4.2 499 585 70 a28 A8 3.6 2.5 7.2 2.8 713 817 69 a29 A7 15.7  1.0 7.2 8.7 570 667 125 a30 A7 15.6  1.1 7.0 8.8 565 661 120 a31 A7 16.2  1.0 7.2 9.1 564 664 118 a32 A9 16.3  1.2 5.6 9.3 571 678 123 Underline means outside the range of the present invention. *Extremely low temperature toughness is the K_(I)C value (converted from J value) in liquid hydrogen (−253° C.). the unit is MPa · √m.

As is apparent from Tables 3 and 4, in Manufacturing Nos. a1 to a16, the tensile strength and the yield stress at room temperature, and the extremely low temperature toughness satisfied the target values.

On the other hand, in Manufacturing No. a17, the C content was small, and in Manufacturing No. a20, the Mn content was small, so that the tensile strength was low and the extremely low temperature toughness had also decreased. In each of Manufacturing Nos. a18, a19, and a21 to a25, the C content, Si content, Mn content, P content, S content, Cr content, and Al content were large, and the extremely low temperature toughness had decreased. In Manufacturing No. a26, the Nb content and the B content were too large, and the extremely low temperature toughness had decreased. In Steel material No. a27, the Ti content and the N content were large, and the extremely low temperature toughness had decreased.

Manufacturing Nos. a28 to a32 are examples in which manufacturing conditions that deviated from preferable ranges are adopted. In Manufacturing No. a28, the heating temperature during rolling was low and the rolling finishing temperature was also low, so that the tensile strength had increased excessively and the extremely low temperature toughness had decreased. In each of Manufacturing Nos. a29, a31, and a32, the heating temperature during rolling, the rolling finishing temperature, and the reheating hardening temperature were high, and the prior austenite grain size had increased, so that the extremely low temperature toughness had decreased. In Manufacturing No. a30, the cumulative rolling reduction at 950° C. or lower was small, and the prior austenite grain size had increased, so that the extremely low temperature toughness had decreased.

Example 2: Ni Steel Having Ni Content of Less than 11.5%

Steel was melted by a converter and slabs having a thickness of 100 mm to 300 mm were manufactured by continuous casting. Tables 5 and 6 show the chemical compositions of Kinds of steel B1 to B24. These slabs were heated, subjected to controlled rolling, directly subjected to water cooling, and subjected to heat treatments including reheating hardening, an intermediate heat treatment, and tempering, whereby steel plates were manufactured. The retention time of the heating of the hot rolling was set to 30 minutes to 120 minutes, and the retention time of the heat treatments including the reheating hardening, the intermediate heat treatment, and the tempering was set to 20 minutes to 60 minutes. Water cooling to 200° C. or lower was performed after the hot rolling. Means for cooling in the heat treatments including the reheating hardening, the intermediate heat treatment, and the tempering was water cooling, and water cooling to 200° C. or lower from the treatment temperature of each of the heat treatments was performed. Samples were taken from the steel plates, and the metallographic structure, tensile properties, and toughness thereof were evaluated.

TABLE 5 Steel Chemical composition (mass %, remainder consists of Fe and impurities) material C Si Mn P S Cu Ni Cr Mo Al Nb Ti B1 0.030 0.18 0.40 0.0040 0.0015 10.7 0.024 0.007 B2 0.060 0.06 0.44 0.0030 0.0017 10.8 0.03 0.01 0.023 B3 0.032 0.19 0.33 0.0020 0.0010 11.0 0.02 0.04 0.011 B4 0.035 0.10 0.30 0.0050 0.0009 0.05 11.4 0.020 0.001 B5 0.042 0.19 0.50 0.0040 0.0007 0.02 11.3 0.04 0.019 B6 0.036 0.14 0.41 0.0040 0.0030 11.2 0.35 0.40 0.017 0.009 B7 0.050 0.10 0.42 0.0030 0.0015 11.2 0.15 0.20 0.050 0.009 B8 0.056 0.19 0.38 0.0060 0.0012 11.3 0.05 0.025 0.015 0.015 B9 0.045 0.16 0.40 0.0050 0.0010 0.50 10.5 0.06 0.010 B10 0.035 0.17 0.41 0.0040 0.0005 10.6 0.038 0.002 B11 0.049 0.06 0.39 0.0030 0.0025 0.03 10.9 0.03 0.045 B12 0.048 0.08 0.43 0.0030 0.0006 0.20 11.1 0.04 0.040 0.012 B13 0.045 0.16 0.44 0.0050 0.0007 0.40 11.0 0.25 0.016 0.010 0.005 Steel Chemical composition (mass %, remainder consists of Fe and impurities) material V B Ca REM N O Note B1 0.0002 0.0020 0.0010 Present B2 0.030 0.0021 0.0007 Invention B3 0.0002 0.0017 0.0024 0.0012 Example B4 0.012 0.0030 0.0014 B5 0.0002 0.0025 0.0012 B6 0.0025 0.0016 B7 0.041 0.0010 0.0005 0.0030 0.0027 0.0009 B8 0.060 0.0016 0.0018 0.0016 B9 0.0020 0.0025 0.0032 0.0020 B10 0.0040 0.0044 0.0011 B11 0.0002 0.0050 0.0015 0.0015 B12 0.0050 0.0010 B13 0.010 0.0026 0.0030 Blank means that no element is intentionally added.

TABLE 6 Steel Chemical composition (mass %, remainder consists of Fe and impurities) material C Si Mn P S Cu Ni Cr Mo Al Nb Ti B14 0.024 0.18 0.48 0.0040 0.0025 11.2 0.14 0.020 0.012 0.012 B15 0.070 0.18 0.45 0.0050 0.0016 11.3 0.030 B16 0.055 0.21 0.44 0.0050 0.0010 11.1 0.015 B17 0.033 0.17 0.20 0.0050 0.0014 10.6 0.030 0.010 0.010 B18 0.052 0.17 0.63 0.0060 0.0025 0.02 11.2 0.20 0.043 0.012 B19 0.055 0.16 0.49 0.0070 0.0020 11.2 0.30 0.038 0.009 B20 0.056 0.16 0.48 0.0030 0.0044 10.8 0.28 0.040 0.010 B21 0.054 0.05 0.38 0.0050 0.0024 11.0 0.43 0.015 0.012 0.010 B22 0.058 0.08 0.48 0.0050 0.0020 0.40 10.8 0.06 0.15 0.060 B23 0.056 0.18 0.48 0.0050 0.0027 0.25 11.2 0.30 0.15 0.040 0.019 B24 0.055 0.19 0.47 0.0050 0.0027 0.15 10.8 0.14 0.15 0.013 0.018 Steel Chemical composition (mass %, remainder consists of Fe and impurities) material V B Ca REM N O Note B14 0.045 0.0002 0.0032 0.0012 Comparative B15 0.0030 0.0014 Example B16 0.0025 0.0034 0.0020 B17 0.035 0.0033 0.0016 B18 0.0002 0.0036 0.0014 B19 0.0039 0.0019 B20 0.035 0.0012 0.0037 0.0015 B21 0.044 0.0015 0.0047 0.0021 B22 0.0048 0.0020 B23 0.0032 0.0046 0.0016 B24 0.0002 0.0032 0.0060 0.0025 Blank means that no element is intentionally added. Underline means outside the range of the present invention.

A method of identifying the metallographic structure of the sample, a method of evaluating the mechanical properties, and acceptance criteria for the mechanical properties were the same as those for the samples disclosed in Tables 1 to 4. Tables 3 and 4 show the plate thickness, manufacturing method, base metal properties, and metallographic structure of steel materials (Manufacturing Nos. b1 to b30) manufactured using slabs having the chemical compositions of Kinds of steel B1 to B24.

TABLE 7 Heating, rolling, and heat treatment conditions Cumulative Water Inter- Heating rolling cooling mediate temperature reduction Rolling start Reheating heat Plate during at 950° C. finishing temperature hardening treatment Tempering Manufacturing Steel thickness rolling or lower temperature after rolling temperature temperature temperature condition No. material (mm) (° C.) (%) (° C.) (° C.) (° C.) (° C.) (° C.) b1 B1 12 1050 95 800 710 760 650 560 b2 B2 18 1070 82 780 680 750 630 540 b3 B3 14 1050 90 800 710 770 640 550 b4 B4 12 1060 88 770 680 740 640 530 b5 B5 18 1030 90 790 720 760 630 530 b6 B6 20 1050 83 650 550 770 630 540 b7 B7 20 1100 80 680 590 780 630 540 b8 B8 20 990 90 770 690 760 630 540 b9 B8 20 1030 83 780 710 760 640 540 b10 B9 18 950 82 780 740 760 630 540 b11 B10 14 970 92 760 700 780 640 540 b12 B11 16 1020 87 770 680 750 630 530 b13 B12 18 980 85 720 630 750 620 530 b14 B13 20 1000 87 700 590 750 630 540 Metallographic structure Average Base metal properties grain Average Volume Average Extremely size of aspect fraction effective low prior γ ratio of of γ grain Yield Tensile temperature Manufacturing Steel grains prior γ phase size stress strength toughness* condition No. material (μm) grains (%) (μm) (MPa) (MPa) (MPa · √m) Note b1 B1 8.0 1.1 2.4 5.0 473 568 220 Present b2 B2 6.8 1.2 4.9 2.5 524 633 179 Invention b3 B3 7.7 1.0 3.5 3.6 486 596 196 Example b4 B4 7.2 1.5 2.3 4.9 460 560 215 b5 B5 7.8 1.1 4.0 3.4 521 635 183 b6 B6 3.0 2.4 3.6 2.3 580 675 150 b7 B7 4.2 1.7 4.6 2.4 566 670 155 b8 B8 6.7 1.9 4.7 3.0 550 659 157 b9 B8 7.5 1.6 2.0 5.5 486 586 151 b10 B9 7.1 1.3 4.0 3.3 507 615 185 b11 B10 4.8 1.7 2.5 4.7 478 577 212 b12 B11 6.7 1.4 4.8 4.4 485 580 204 b13 B12 5.5 2.2 5.0 4.0 537 634 170 b14 B13 4.3 2.3 4.2 2.0 564 650 166 *Extremely low temperature toughness is the K_(I)C value (converted from J value) in liquid hydrogen (−253° C.), the unit is MPa · √m.

TABLE 8 Heating, rolling, and heat treatment conditions Cumulative Water Inter- Heating rolling cooling mediate temperature reduction Rolling start Reheating heat Plate during at 950° C. finishing temperature hardening treatment Tempering Manufacturing Steel thickness rolling or lower temperature after rolling temperature temperature temperature condition No. material (mm) (° C.) (%) (° C.) (° C.) (° C.) (° C.) (° C.) b15 B14 20 1040 83 790 720 780 640 540 b16 B15 20 1050 83 780 710 750 630 540 b17 B16 20 1080 83 800 740 770 640 550 b18 B17 20 1080 83 790 720 780 650 560 b19 B18 20 1100 83 800 740 770 630 550 b20 B19 20 1050 83 780 710 770 630 550 b21 B20 20 1080 83 790 700 770 640 550 b22 B21 20 1070 83 790 730 760 630 550 b23 B22 20 1080 83 770 710 770 630 550 b24 B23 20 1050 83 760 690 770 630 550 b25 B24 20 1060 83 770 690 770 640 550 b26 B12 18 930 85 630 600 750 620 530 b27 B1 12 1130 90 800 750 760 650 560 b28 B5 20 1090 75 790 730 760 630 530 b29 B6 20 1080 83 830 770 770 630 540 b30 B7 20 1080 80 700 630 800 630 540 Metallographic structure Average Base metal properties grain Average Volume Average Extremely size of aspect fraction effective low prior γ ratio of of γ grain Yield Tensile temperature Manufacturing Steel grains prior γ phase size stress strength toughness* condition No. material (pm) grains (%) (μm) (MPa) (MPa) (MPa · √m) Note b15 B14 7.2 1.5 1.2 4.7 470 564 98 Comparative b16 B15 6.4 1.2 4.6 4.3 480 573 116 Example b17 B16 7.1 1.0 3.5 4.6 470 566 113 b18 B17 6.5 1.1 1.6 4.8 465 563 115 b19 B18 6.8 1.5 4.7 3.2 526 627 120 b20 B19 6.7 1.4 3.4 3.4 522 625 112 b21 B20 6.8 1.2 3.2 3.4 530 638 105 b22 B21 6.9 1.6 4.2 3.3 532 643 124 b23 B22 4.0 1.3 3.6 3.1 550 650 130 b24 B23 6.7 2.5 3.4 2.3 570 674 116 b25 B24 3.7 1.2 3.5 2.8 560 667 82 b26 B12 3.4 2.5 4.3 2.1 590 695 80 b27 B1 9.2 1.0 2.6 5.6 485 578 105 b28 B5 8.6 1.1 4.1 5.0 535 640 124 b29 B6 8.5 1.4 3.9 5.5 580 678 115 b30 B7 8.7 1.5 4.5 5.3 570 675 120 Underline means outside the range of the present invention. *Extremely low temperature toughness is the K_(I)C value (converted from J value) in liquid hydrogen (−253° C.), the unit is MPa · √m.

As is apparent from Tables 7 and 8, in Manufacturing Nos. b1 to b14, the tensile strength and the yield stress at room temperature, and the extremely low temperature toughness satisfied the target values.

On the other hand, in Manufacturing No. b15, the C content was small, and in Manufacturing No. b18, the Mn content was small, so that the extremely low temperature toughness had decreased. In each of Manufacturing Nos. b16, b17, and b19 to b23, the C content, Si content, Mn content, P content, S content, Cr content, and Al content were large, and the extremely low temperature toughness had decreased. In Manufacturing No. b24, the Nb content and the B content were too large, and the extremely low temperature toughness had decreased. In Manufacturing No. b25, the Ti content and the N content were large, and the extremely low temperature toughness had decreased.

Manufacturing Nos. b26 to b30 are examples in which manufacturing conditions that deviated from preferable ranges are adopted. In Manufacturing No. b26, the heating temperature during rolling was low and the rolling finishing temperature was also low, so that the tensile strength had increased excessively and the extremely low temperature toughness had decreased. In each of Manufacturing Nos. b27, b29, and b30, the heating temperature during rolling, the rolling finishing temperature, and the reheating hardening temperature were high, the prior austenite grain size had increased, and the effective grain size had also increased, so that the extremely low temperature toughness had decreased. In Manufacturing No. b28, the cumulative rolling reduction at 950° C. or lower was small, and the prior austenite grain size had increased, so that the extremely low temperature toughness had decreased. 

The invention claimed is:
 1. A nickel-containing steel comprising, as a chemical composition, by mass %: C: 0.030% to 0.070%; Si: 0.03% to 0.30%; Mn: 0.20% to 0.80%; Ni: 10.5% to 12.4%; Al: 0.010% to 0.060%; N: 0.0015% to 0.0060%; O: 0.0007% to 0.0030%; Cu: 0% to 0.50%; Cr: 0% to 0.50%; Mo: 0% to 0.40%; Nb: 0% to 0.020%; V: 0% to 0.080%; Ti: 0% to 0.020%; B: 0% to 0.0020%; Ca: 0% to 0.0040%; REM: 0% to 0.0050%; P: 0.0080% or less; S: 0.0040% or less; and a remainder: Fe and impurities, wherein a metallographic structure of a thickness middle plane contains 2.0 vol % to 20.0 vol % of an austenite phase, an average grain size of prior austenite grains, measured in accordance with JIS G 0551, measured in a section of the thickness plane parallel to a rolling direction and a thickness direction is 3.0 μm to 15.0 μm, an average aspect ratio of the prior austenite grains, wherein the aspect ratio of the prior austenite grains is defined as: length of the prior austenite grains in a rolling direction/thickness of the prior austenite grains in the thickness direction, measured in the section of the thickness middle plane parallel to the rolling direction and the thickness direction is 1.0 to 2.4, a plate thickness is 4.5 mm to 20 mm, when the nickel-containing steel contains Ni: less than 11.5%, the nickel-containing steel contains C: 0.060% or less, Si: 0.19% or less, Mn: 0.30% to 0.50%, A1: 0.050% or less, N: 0.0050% or less, Cr: 0.35% or less, Nb: 0.015% or less, V: 0.060% or less, Ti: 0.015% or less, P: 0.0060% or less, and S: 0.0030% or less, and the average grain size of the prior austenite grains, measured in accordance with JIS G 0551, is 8.0 μm or less, a yield stress at room temperature, measured in accordance with JIS Z 2241, is 460 MPa to 710 MPa, and a tensile strength at the room temperature, measured in accordance with JIS Z 2241, is 560 MPa to 810 MPa.
 2. The nickel-containing steel according to claim 1 comprising, as the chemical composition, by mass %: Ni: 11.5% or more, and Mn: 0.50% or less.
 3. The nickel-containing steel according to claim 1 comprising, as the chemical composition, by mass %: Ni: 11.5% or more, wherein the average grain size of the prior austenite grains, measured in accordance with JIS G 0551, is 9.0 μm or less.
 4. The nickel-containing steel according to claim 1, wherein an average effective grain size measured in the section of the thickness middle plane parallel to the rolling direction and the thickness direction is 2.0 μm to 8.0 μm, wherein the average effective grain size is measured by taking a sample from the steel after tempering, and observing five or more visual fields at a magnification of 2,000-fold using an electron backscatter diffraction analyzer, wherein an effective grain is defined as a grain surrounded by a grain boundary, and a grain boundary is defined as a boundary of a metallographic structure having an orientation difference of 15° or more, and wherein a circle equivalent grain size is obtained from multiple areas of effective grains by image processing, and an average value of an obtained circle equivalent grain sizes represents the average effective grain size.
 5. The nickel-containing steel according to claim 1, wherein an average effective grain size measured in the section of the thickness middle plane parallel to the rolling direction and the thickness direction is 2.0 μm to 5.0 μm, wherein the average effective grain size is measured by taking a sample from the steel after tempering, and observing five or more visual fields at a magnification of 2,000-fold using an electron backscatter diffraction analyzer, wherein an effective grain is defined as a grain surrounded by a grain boundary, and a grain boundary is defined as a boundary of a metallographic structure having an orientation difference of 15° or more, and wherein a circle equivalent grain size is obtained from multiple areas of effective grains by image processing, and an average value of an obtained circle equivalent grain sizes represents the average effective grain size.
 6. The nickel-containing steel according to claim 2 comprising, as the chemical composition, by mass %: Ni: 11.5% or more, wherein the average grain size of the prior austenite grains, measured in accordance with JIS G 0551, is 9.0 μm or less.
 7. The nickel-containing steel according to claim 2, wherein an average effective grain size measured in the section of the thickness middle plane parallel to the rolling direction and the thickness direction is 2.0 μm to 8.0 μm, wherein the average effective grain size is measured by taking a sample from the steel after tempering, and observing five or more visual fields at a magnification of 2,000-fold using an electron backscatter diffraction analyzer, wherein an effective grain is defined as a grain surrounded by a grain boundary, and a grain boundary is defined as a boundary of a metallographic structure having an orientation difference of 15° or more, and wherein a circle equivalent grain size is obtained from multiple areas of effective grains by image processing, and an average value of an obtained circle equivalent grain sizes represents the average effective grain size.
 8. The nickel-containing steel according to claim 3, wherein an average effective grain size measured in the section of the thickness middle plane parallel to the rolling direction and the thickness direction is 2.0 μm to 8.0 μm, wherein the average effective grain size is measured by taking a sample from the steel after tempering, and observing five or more visual fields at a magnification of 2,000-fold using an electron backscatter diffraction analyzer, wherein an effective grain is defined as a grain surrounded by a grain boundary, and a grain boundary is defined as a boundary of a metallographic structure having an orientation difference of 15° or more, and wherein a circle equivalent grain size is obtained from multiple areas of effective grains by image processing, and an average value of an obtained circle equivalent grain sizes represents the average effective grain size.
 9. The nickel-containing steel according to claim 6, wherein an average effective grain size measured in the section of the thickness middle plane parallel to the rolling direction and the thickness direction is 2.0 μm to 8.0 μm, wherein the average effective grain size is measured by taking a sample from the steel after tempering, and observing five or more visual fields at a magnification of 2,000-fold using an electron backscatter diffraction analyzer, wherein an effective grain is defined as a grain surrounded by a grain boundary, and a grain boundary is defined as a boundary of a metallographic structure having an orientation difference of 15° or more, and wherein a circle equivalent grain size is obtained from multiple areas of effective grains by image processing, and an average value of an obtained circle equivalent grain sizes represents the average effective grain size.
 10. The nickel-containing steel according to claim 2, wherein an average effective grain size measured in the section of the thickness middle plane parallel to the rolling direction and the thickness direction is 2.0 μm to 5.0 μm, wherein the average effective grain size is measured by taking a sample from the steel after tempering, and observing five or more visual fields at a magnification of 2,000-fold using an electron backscatter diffraction analyzer, wherein an effective grain is defined as a grain surrounded by a grain boundary, and a grain boundary is defined as a boundary of a metallographic structure having an orientation difference of 15° or more, and wherein a circle equivalent grain size is obtained from multiple areas of effective grains by image processing, and an average value of an obtained circle equivalent grain sizes represents the average effective grain size.
 11. The nickel-containing steel according to claim 3, wherein an average effective grain size measured in the section of the thickness middle plane parallel to the rolling direction and the thickness direction is 2.0 μm to 5.0 μm, and wherein the average effective grain size is measured by taking a sample from the steel after tempering, and observing five or more visual fields at a magnification of 2,000-fold using an electron backscatter diffraction analyzer, wherein an effective grain is defined as a grain surrounded by a grain boundary, and a grain boundary is defined as a boundary of a metallographic structure having an orientation difference of 15° or more, and wherein a circle equivalent grain size is obtained from multiple areas of effective grains by image processing, and an average value of an obtained circle equivalent grain sizes represents the average effective grain size.
 12. The nickel-containing steel according to claim 6, wherein an average effective grain size measured in the section of the thickness middle plane parallel to the rolling direction and the thickness direction is 2.0 μm to 5.0 μm, wherein the average effective grain size is measured by taking a sample from the steel after tempering, and observing five or more visual fields at a magnification of 2,000-fold using an electron backscatter diffraction analyzer, wherein an effective grain is defined as a grain surrounded by a grain boundary, and a grain boundary is defined as a boundary of a metallographic structure having an orientation difference of 15° or more, and wherein a circle equivalent grain size is obtained from multiple areas of effective grains by image processing, and an average value of an obtained circle equivalent grain sizes represents the average effective grain size. 